If you need to increase the rate of power coming from a battery bank, connect your batteries in series. For example, three 12-volt, 200 amp-hour batteries connected in series produce 36 volts at 200 amp hours of use. Voltage is a measurement of the rate that energy moves from the batteries. If electricity were a car, the voltage would be its speed. However, if you need to increase the amount of capacity available to you, a parallel battery connection is the way to go. Using the prior example, three 12-volt, 200 amp-hour batteries connected in parallel still only produce 12 volts of electricity but will do so for 600 amp hours. Amperage is a measure of the power available. In the car scenario, amperage would be like gasoline. Multiple batteries connected in parallel are useful on boats or recreational vehicles that don't have access to shore power. Depending on your amperage needs, a parallel-connected battery bank can keep your lights on and your refrigerator running for hours or even days.
Choose the right batteries. In order for a parallel connection to operate safely and efficiently, the batteries you use in the battery bank must all be of the same type wet-cell or gel cell, deep-cycle or cranking and close in age. Older batteries are often worn and differ significantly in capacity from original specifications. Deep cycle, gel cell batteries work best for long-term, efficient energy storage.
Select the correct cables. Use different colors for positive and negative; red is a very common color for positive cables, whereas black or white are typically used to indicate the negative current. For a 12-volt electrical system on a boat or camper, use wires between 10 and 14 gauge. Wire of this gauge can handle the voltage from the batteries with little enough resistance to ensure your electrical system operates efficiently.
Connect the positive battery posts. Connect the wire or cable you've selected for the positive current from the positive post of the first battery to the positive post of the second battery. If you are using more than two batteries, continue until the positive posts of each battery are connected to the positive post of the battery immediately before it. Batteries on the ends of the bank should only have one wire connected to their positive posts; batteries in the middle should have two wires connected to their positive posts: one from the battery before and one to the battery after.
Connect the negative battery posts. Just like the positive posts, connect the wire or cable you've selected for the negative current from the negative post of the first battery to the negative post of the second battery, from the second battery to third, from the third battery to the fourth, and so on. Again, batteries on the ends of the bank should only have one wire connected to their negative posts; batteries in the middle should have two wires connected to their negative posts: one from the battery before and one to the battery after.
Connect the battery bank to your electrical system. From one of the end batteries, connect the positive and negative posts to the fuse or breaker panel of your 12-volt electrical system. The life of the batteries' charge depends on the number of amps you use and the amount of time you draw from the battery. Three 200-amp hour batteries, 600 total amp hours, for example, power a television that draws 100 amps for six hours. Different devices obviously draw different amperages. If you ran nothing but lights, which are relatively efficient, the system charge could last for days. Throw in a blow-dryer and the life of the system charge is drastically reduced.